Various electric accessory loads can be operated in a vehicle, using the vehicle electrical system to supply the electrical power requirements of the load. Such loads may be in the form of permanently installed devices, as in the case of factory-installed equipment, or portable devices, as in the case of consumer equipment that is powered through a vehicle accessory load connector or cigarette lighter plug. Although an engine-driven alternator develops current for both charging the storage battery and supplying power to electrical loads during vehicle operation, many electrical loads consume sufficient power to discharge the vehicle storage battery if left on during a prolonged period of vehicle inactivity. Accordingly, the electrical systems in some vehicles have been designed to automatically disconnect the accessory power supply after the engine has been turned off for a predefined interval and/or if the ignition switch is off and the battery voltage falls below a reference value. See, for example, the U.S. Pat. No. 4,493,001, which is assigned to General Motors Corporation, and the earlier issued patents mentioned therein. However, many vehicle electrical systems do not have such a safeguard against battery discharging, and what is needed is an accessory load power supply circuit that will protect against battery discharging in otherwise unprotected vehicles.